The various features and multiple structural levels of semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements. In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response, more complex optical tools have been developed. For example, tools with multiple angles of illumination, shorter and broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
Another response to these recent challenges has been the adoption of x-ray metrology for measurements including film thickness, composition, strain, surface roughness, line edge roughness, and porosity. Many x-ray metrology techniques used in semiconductor manufacturing can benefit from high brightness x-ray sources. For example, critical dimension small angle x-ray scattering (CD-SAXS) measurements often require long integration times due to the low scattering of certain materials. A high brightness source can improve the throughput of CD-SAXS measurements.
X-ray sources including electron beam sources with water cooled targets and solid, rotating anodes have been employed. Another exemplary high brightness X-ray source is a liquid metal jet X-ray source having a liquid metal anode. Unfortunately, for both conventional solid and liquid anode sources, measurement throughput has been impaired by limited power loading on the anode. An increase in power loading of a conventional solid metal anode source causes ablation and destruction of the anode. For typical liquid metal anode sources, an increase in power loading produces excessive metal vapor that damages the cathode. Low melting temperature anode materials have very high metal vapor pressure. Thus, the cathode of the electron gun of the liquid metal jet x-ray source is susceptible to poisoning by liquid metal vapor.
Liquid metal jet sources employ low melting point metals or alloys of low melting point metals. These metals are usually low atomic number elements that emit relatively low energy x-rays (i.e., long wavelength x-rays). These liquid metal anode materials are selected for their low melting point and are maintained in the liquid phase and recollected by a liquid circulation system. This approach is limited to metals or metal alloys that can be sustained in a liquid phase. Thus, the low melting temperature requirement severely limits the available energy range of a liquid metal jet x-ray source suitable for semiconductor metrology.
Future metrology applications present challenges for metrology due to increasingly high resolution requirements, multi-parameter correlation, increasingly complex geometric structures, and increasing use of opaque materials. The adoption of x-ray metrology for semiconductor applications requires improved x-ray sources with the highest possible brightness.